Brigo, Laura (2008) Water slippage over micro and nano structured surfaces. [Ph.D. thesis]
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The subject of this research is the systematic study of the dependence of water slippage inside microfluidic channels on the morphology and chemical nature of the solid wall.
A low-cost Micro Particle Imaging Velocimetry (u-PIV) apparatus has been designed and developed to perform the experimental characterization of the microflow dynamics.
The set-up is sufficiently versatile to image a pretty wide range of flow conditions present in microfluidic devices, allowing the determination of fluid velocity fields both in the bulk of the fluid by epifluorescence microscopy and at the interface with a transparent solid surface by the use of total internal reflection fluorescence microscopy (TIRFM).
We initially tested our apparatus against the recent results obtained by leading groups in the application of the u-PIV technique to the study of liquid-solid boundary conditions. In particular, we measured water slip length on flat surfaces as a function of the liquid-solid contact angle.
Then, we investigated the water slip dynamics in microchannels presenting a patterned floor.
Measurements on isotropically patterned surfaces, presenting a regular grid of pillars or dots as to model superficial roughness on the micron scale, generally showed a decrease in the value of the slip length with respect to the flat substrate.
Measurements performed as a function of the orientation of micrometric canals impressed on the floor of a microchannel with respect to the main flux direction, lead to new interesting experimental evidences. A confirmation of a reduction in the value of the slip length due to wall micropattering was obtained and, moreover, the effect that the anisotropy of a pattern on the floor has on the flow direction was investigated. By changing the orientation and the modulation of linear microstructures on a solid wall, it was possible to modify the flow direction even inside planar lithographic-made microchannels with a constant pressure gradient applied at both ends.
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